Grinders, shredders or mills are well known devices for reducing the particle size of a material. For example, U.S. Pat. No. 5,192,029 to Harris and U.S. Pat. No. 5,680,994 to Eide et al. each disclose mills for grinding garbage. Each of these mills includes a rotor rotatably mounted in a generally octagonal housing. The rotor includes a generally vertical shaft and a plurality of blades or hammers mounted on the shaft. Garbage is admitted into the housing through an inlet near the top of the housing and is impacted by the blades of the rotor. Material of a reduced particle size is removed from the mill through an outlet near the bottom of the housing. The ground garbage can be sent to a landfill where it will take up less room than unprocessed garbage, or it can be composted or recycled, depending on the included materials. If the material is to be shipped, it can be shipped more efficiently due to its reduced size and greater density.
The mill of Eide et al. '994 further includes a fan or impeller which is mounted on the rotor shaft below the cutting blades. The fan is intended to create airflow which acts to move material through the mill and to expel it from the outlet. The fan generally comprises a fan disc mounted to the rotor shaft which has a plurality of radially extending lengths of angle iron mounted thereon. One flange of each angle iron is bolted to the fan disc and the other extends upwardly from the disc to act as a fan blade. The angle irons are fixedly mounted to the fan disc and no means are provided for adjusting the airflow for different materials or grinding conditions.
It should be noted that, in addition to moving material through the mill, the airflow from the fan also acts to remove moisture from the material as it is being ground. Since different materials and different grinding conditions produce different moisture levels in the material, it would be advantageous if the rate of airflow could be adjusted.